Medical and other instruments and devices (16, 44) may have a build-up of biological residue film, even after sterilizing. Dead cell membranes in this film can give off endotoxins. To check for the presence of biological residue film, light from a source (10, 40) travels along optical fibers (12, 42) and is focused by a lens (24) on a surface (26) to be examined. Reflected or transmitted light is conveyed by optical fibers (46) to an opto-electrical analyzing device (30, 48). In one embodiment, the opto-electrical device (30) senses the intensity of reflected light to provide an indication of reflectivity attributable to the biological film build-up. In another embodiment, a spectrophotometer (48) converts the returned light into an indication of the reflected spectrum which is analyzed (50) to determine the nature of the material which reflected the light, in particular the type of protein or other biological residue found on the examined surface.